In an earlier contribution to this discussion,
wrote:

I would like to fit a garbage disposal to my new sink. I know they are
supposed to mince up fine enough to go into a drain. I will have a
gulley outside the kitchen (useful for garden stuff), and I could
connect sink/garbage to that, but is there any reason why I cannot
have a small soil pipe stack terminating under the kitchen sink and
connect sink/garbage to that, even though there is no toilet ? Would
I need an air admittance valve ? It strikes me that having such a
stack around would also be useful for future modifications.
Cheers,
Simon.

I don't see why you shouldn't do it if you want to - after all, many kitchen
sinks are connected to stacks rather than going directly into a gulley.

My sink and waste disposal unit both discharge into a horizontal manifold
under the sink whose output is connected to a U-bend, and thence out through
the wall to a gulley.

After having to deal with a number of blockages, I re-jigged the pipework to
ensure that the waste disposal unit discharges into the manifold directly
opposite the oulet - so as to have as straight a path as possible to the
drain.
--
Cheers,
Set Square
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